[PLAT-6531] Added periodic yield curve with spread
[PLAT-6531] Changes from code review, no functional changes
[PLAT-6531] Removed final from method parameters and arguments
[PLAT-6531] Added null check for curve and curveFixed constructor arguments
Merge pull request #69 from OpenGamma/topic/PLAT-6531

[PLAT-6531] Added periodic yield curve with spread